The present invention relates generally to television signal encoding and decoding systems and particularly concerns an improvement to a technique involving video signal inversion and reinversion in the RF domain.
In subscription or pay television systems programming signals are typically transmitted, either over-the-air or through a suitable cable network, in an encoded or scrambled form rendering the broadcast video information essentially unviewable when received and reproduced by a conventional television receiver. In order to decode or unscramble the video display, each system subscriber is provided with a decoder operable for decoding the broadcast signals and for coupling the decoded signals to a conventional television receiver for viewing.
One known technique for encoding a broadcast television signal contemplates inverting the video information by adding to the television signal carrier an encoding wave which is equal in frequency to but oppositely phased and larger than the original carrier. As a result of the inversion of the video information, a conventional television receiver is incapable of synchronizing to and properly reproducing the original signal. In order to decode the broadcast signal, a decoder is provided for implementing an inverse operation wherein a decoding wave is added to the received signal, the decoding wave having a frequency equal to but again oppositely phased with respect to the received carrier. The thusly reinverted signal may subsequently be coupled to a conventional television receiver for viewing by the subscriber. U.S. Pat. No. 3,982,062 to Simons is exemplary of such RF domain inversion and reinversion systems.
In systems of the foregoing type, it has heretofore been extremely difficult to maintain the desired depth or percentage of modulation in the decoded signal. In particular, in order to faithfully reproduce the original signal it is desired that the percentage of modulation characterizing the encoded broadcast signal be substantially maintained in the decoded signal. Deviations in the percentage of modulation of the decoded signal from the nominal level characterizing the encoded broadcast signal lead to undesired compression or expansion of the decoded signal and thereby to a distorted image when the signal is reproduced by the subscriber's television receiver.
The percentage of modulation of the decoded signal is largely a function of the level of the decoding wave combined with the received signal in the decoder. The level of the decoding wave is therefore normally fixed by suitably setting the gain of an amplifier or attenuator so that the percentage of modulation of the decoded signal will be the same as that characterizing the encoded broadcast signal. However, with time the setting of the amplifier or attenuator will tend to drift varying the level of the decoding wave and thereby the percentage of modulation of the decoded signal, such variations leading to the image distortions described above.